"Capture group reference appears to be unused" with integer names

Hello,
I have noticed `checker.go` using integers to reference unused capture groups (together with the expected names). I can't quite understand why since AFAICS all capture groups are named with strings, this is the program I'm testing:

```
counter nginx_http_requests by host, vhost, method, code
counter nginx_http_requests_cache by host, vhost, cache_status

/(?P[-0-9A-Za-z._:]+) nginx_access: (?P[-0-9A-Za-z._:]+) \S+ (?P[0-9a-f\.:]+) - - \[[^\]]+\] "(?P[A-Z]+) (?P\S+) (?PHTTP\/[0-9\.]+)" (?P\d{3}) ((?P\d+)|-) "[^"]*" "[^"]*" ([-0-9A-Za-z._:]+) ((?P\d+\.\d+)|-) (?P\d+)\.(?P\d+) (?P\S+)/ {

nginx_http_requests[$hostname][$vhost][$request_method][$status]++
nginx_http_requests_cache[$hostname][$vhost][$cache_status]++
}
```

And rc47 shows this output:

```
mtail[367499]: I0905 15:15:34.294884 367499 main.go:114] mtail version 3.0.0-rc47 git revision 5e0099f843e4e4f2b7189c21019de18eb49181bf go version go1.16.5 go arch amd64 go os linux
mtail[367499]: I0905 15:15:34.295490 367499 main.go:115] Commandline: ["/usr/bin/mtail" "--progs" "/etc/mtail" "--logtostderr" "--port" "3903" "--poll_interval" "0" "--logs" "/dev/fd/3"]
mtail[367499]: I0905 15:15:34.295633 367499 main.go:145] no poll interval specified; defaulting to 250ms poll
mtail[367499]: I0905 15:15:34.296227 367499 store.go:182] Starting metric store expiry loop every 1h0m0s
mtail[367499]: I0905 15:15:34.296629 367499 checker.go:253] capture group reference `http_version' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296644 367499 checker.go:253] capture group reference `request_seconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296648 367499 checker.go:253] capture group reference `request_milliseconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296652 367499 checker.go:253] capture group reference `request_uri' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296655 367499 checker.go:253] capture group reference `ups_resp_seconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296658 367499 checker.go:253] capture group reference `remote_addr' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296661 367499 checker.go:253] capture group reference `remote_addr' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296664 367499 checker.go:253] capture group reference `request_uri' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296668 367499 checker.go:253] capture group reference `response_size' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296671 367499 checker.go:253] capture group reference `response_size' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296674 367499 checker.go:253] capture group reference `request_seconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296677 367499 checker.go:253] capture group reference `http_version' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296680 367499 checker.go:253] capture group reference `8' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296683 367499 checker.go:253] capture group reference `request_milliseconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296687 367499 checker.go:253] capture group reference `10' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296690 367499 checker.go:253] capture group reference `ups_resp_seconds' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296693 367499 checker.go:253] capture group reference `11' at nginx.mtail:4:1-399 appears to be unused
mtail[367499]: I0905 15:15:34.296764 367499 runtime.go:188] Loaded program nginx.mtail
mtail[367499]: I0905 15:15:34.296775 367499 runtime.go:84] unmarking nginx.mtail
mtail[367499]: I0905 15:15:34.296780 367499 runtime.go:84] unmarking systemd.mtail_
mtail[367499]: I0905 15:15:34.296812 367499 logstream.go:61] Parsed url as /dev/fd/3
mtail[367499]: I0905 15:15:34.296833 367499 tail.go:282] Tailing /dev/fd/3
mtail[367499]: I0905 15:15:34.297620 367499 mtail.go:126] Listening on [::]:3903
```

Specifically capture groups `8` / `10` / `11`

Thank you !